Asynchronous hybrid ARQ process indication in a MIMO wireless communication system

ABSTRACT

Asynchronous Hybrid Automatic Repeat reQuest (ARQ) process identities are transmitted in a wireless communication system. A linking scheme is established between at least two sets of process identities of two respective corresponding codewords. When a first process identity is selected from among a first set of process identities of a first codeword, a second process identity may be derived in dependence upon the first process identity and the established linking scheme. Finally, a first packet from the first codeword is transmitted using a first transmission channel indicated by the first process identity, and a second packet is transmitted from the second codeword using a second transmission channel indicated by the second process identity. In addition, a control message including only the first process identity is transmitted.

CLAIM OF PRIORITY

This application is a continuation of U.S. Non-Provisional patentapplication Ser. No. 14/021,993 filed Sep. 9, 2013 and entitledASYNCHRONOUS HYBRID ARQ PROCESS INDICATION IN A MIMO WIRELESSCOMMUNICATION SYSTEM, now U.S. Pat. No. 9,071,434, which is acontinuation of U.S. Non-Provisional patent application Ser. No.12/222,113 filed Aug. 1, 2008 and entitled ASYNCHRONOUS HYBRID ARQPROCESS INDICATION IN A MIMO WIRELESS COMMUNICATION SYSTEM, now U.S.Pat. No. 8,553,624, and claims priority to U.S. Provisional PatentApplication No. 60/960,709 filed Oct. 10, 2007 and entitled ASYNCHRONOUSHYBRID ARQ PROCESS INDICATION IN A MIMO WIRELESS COMMUNICATION SYSTEM.The content of the above-identified documents is incorporated herein byreference.

BACKGROUND

Field of the Disclosure

The present disclosure relates to transmitting Asynchronous HybridAutomatic Repeat request (ARQ) process identities in a wirelesscommunication system.

Description of the Related Art

During data transmission, especially wireless data transmission, errorinevitably occurs to decrease the quality of the transmitted data.Therefore, the data is retransmitted in order to correct the error.

Automatic Repeat-reQuest (ARQ) is an error control method for datatransmission which makes use of acknowledgements and timeouts to achievereliable data transmission. An acknowledgement is a message sent by thereceiver to the transmitter to indicate that it has correctly received adata frame.

Usually, when the transmitter does not receive the acknowledgementbefore the timeout occurs (i.e., within a reasonable amount of timeafter sending the data frame), the transmitter retransmits the frameuntil the data within the frame is either correctly received or theerror persists beyond a predetermined number of re-transmissions.

Hybrid ARQ (HARQ) is a variation of the ARQ error control method, whichgives better performance than the ordinary ARQ scheme, particularly overwireless channels, at the cost of increased implementation complexity.One version of HARQ is described in the IEEE 802.16e standard.

The HARQ protocol can be further classified into a synchronous HARQprotocol and an asynchronous HARQ protocol. In the synchronous HARQprotocol, the retransmissions happen at fixed time intervals and controlinformation only needs to be transmitted along with a first subpackettransmission. The drawback of synchronous HARQ, however, is that theretransmission subpackets cannot be scheduled at preferable channelconditions because the timing of the retransmission is predetermined.Also, the modulation, coding and resource format cannot be adapted atthe time of retransmission according to the prevailing channelconditions at the time of retransmission.

In the asynchronous HARQ protocol, the retransmission timing,modulation, coding and resource format can be adapted according to theprevailing channel and resource conditions at the time ofretransmission. The control information, however, needs to be sent alongwith all the subpackets. The control information transmission along witheach subpacket allows adjusting the transmission timing, modulation,coding and resources allocated.

In the 3rd Generation Partnership Project (3GPP) Long Term Evolution(LTE) systems, a maximum of two codewords are used for transmission oftwo, three or four MIMO layers. In addition, an HARQ process identity isused to indicate the ID of the channel in an N-channel HARQ system. Forexample, a 3-bit process ID allows simultaneous operation on 8 SAWchannels.

When two subpackets from two respectively corresponding codewords aretransmitted using the HARQ transmission scheme, the transmission rankmay change from 2 to 1 at time of retransmission. If both subpacketsused a process ID of 0 (PID=0) at the first transmission in rank-2, onlya single codeword can be retransmitted in rank-1. This is because asingle subpacket under a single PID can be retransmitted in rank-1. Thesecond codeword transmission has to start from the beginning at a latertime. This results in loss of the previously transmitted subpacket inrank-2.

When two subpackets from two respectively corresponding codewords aretransmitted using the HARQ transmission scheme, the transmission rankmay also change from 1 to 2 at time of retransmission. If a firstsubpacket uses a process ID of 0, while a second subpacket uses aprocess ID of 1 at the first transmission in rank-1, the two codewordsare transmitted in rank-1 in two subframes because a single codeword canbe transmitted in rank-1 in a given subframe. We note that theretransmissions for the two codewords can be performed in rank-2 becausethe two codewords are transmitted on different hybrid ARQ processes.

SUMMARY

It is therefore an object of the present disclosure to provide animproved method and apparatus for wireless communication.

It is another object of the present disclosure to provide an improvedmethod and apparatus for efficiently transmitting Hybrid AutomaticRepeat-reQuest (HARQ) process identities.

According to one aspect of the present disclosure, a linking scheme isestablished between at least two sets of process identities of tworespective corresponding codewords. When a first process identity isselected from among a first set of process identities of a firstcodeword, a second process identity may be derived in dependence uponthe first process identity and the established linking scheme. Finally,a first packet from the first codeword is transmitted using a firsttransmission channel indicated by the first process identity, and asecond packet is transmitted from the second codeword using a secondtransmission channel indicated by the second process identity. Inaddition, a control message including only the first process identity istransmitted.

The control message may also include a codeword to layer mapping fieldindicating the mapping for the codewords to transmission layers.

The first packet and the second packet may be transmitted on differentfrequency subbands.

According to another aspect of the present disclosure, a linking schemeis established between a certain set of process identity fields and atleast two sets of process identities of two respective correspondingcodewords. When a process identity field is selected from among thecertain set of process identity fields, a first process identity and asecond process identity may be derived in dependence upon the selectedprocess identity field and the established linking scheme. Finally, afirst packet from the first codeword is transmitted using a firsttransmission channel indicated by the first process identity, and asecond packet is transmitted from the second codeword using a secondtransmission channel indicated by the second process identity. Inaddition, a control message including the selected process identityfield is transmitted.

According to still another aspect of the present disclosure, a linkingscheme is established between a certain set of process identity fields,a certain set of differential process identities, and at least two setsof process identities of two respective corresponding codewords.Therefore, when a process identity field is selected from among thecertain set of process identity fields, and a differential processidentity is selected from among the certain set of differential processidentities, a first process identity and a second process identity maybe derived in dependence upon the selected process identity field, theselected differential process identity and the established linkingscheme. Finally, a first packet from the first codeword is transmittedusing a first transmission channel indicated by the first processidentity, and a second packet is transmitted from the second codewordusing a second transmission channel indicated by the second processidentity. In addition, a control message including the selected processidentity field and the selected differential process identity istransmitted.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the subject matter of the presentdisclosure, and many of the attendant advantages thereof, will bereadily apparent as the same becomes better understood by reference tothe following detailed description when considered in conjunction withthe accompanying drawings in which like reference symbols indicate thesame or similar components, wherein:

FIG. 1 schematically illustrates an Orthogonal Frequency DivisionMultiplexing (OFDM) transceiver chain;

FIG. 2 schematically illustrates a scheme for generating subpackets;

FIG. 3 schematically illustrates an example of Hybrid ARQ scheme in awireless communication system;

FIG. 4 schematically illustrates a synchronous Hybrid ARQ scheme;

FIG. 5 schematically illustrates an asynchronous Hybrid ARQ scheme;

FIG. 6 schematically illustrates a Multiple Input Multiple Output (MIMO)transceiver chain;

FIG. 7 schematically illustrates a Single-code word MIMO scheme;

FIG. 8 schematically illustrates a Multi-code word MIMO scheme;

FIG. 9 schematically illustrates Multi-code word MIMO scheme for2-layers transmission in the 3GPP LTE system;

FIG. 10 schematically illustrates Multi-code word MIMO scheme for3-layers transmission in the 3GPP LTE system;

FIG. 11 schematically illustrates Multi-code word MIMO scheme for4-layers transmission in the 3GPP LTE system;

FIG. 12 schematically illustrates an 8-channel Asynchronous Hybrid ARQscheme;

FIG. 13 schematically illustrates an example of subpackets from twocodewords;

FIG. 14 schematically illustrates an example of HARQ retransmission whenrank changes from 2 to 1 at the time of retransmissions;

FIG. 15 schematically illustrates an example of HARQ retransmission whenrank changes from 1 to 2 at time of retransmissions;

FIG. 16 schematically illustrates an example of HARQ retransmissions forthe case when rank changes from 2 to 1 at time of retransmissions as afirst embodiment according to the principles of the present disclosure;

FIG. 17 schematically illustrates an example of HARQ retransmissions forthe case when rank changes from 1 to 2 at time of retransmissions as asecond embodiment according to the principles of the present disclosure;

FIG. 18 schematically illustrates an example of HARQ retransmissions forthe case when rank changes from 1 to 2 at time of retransmissions as athird embodiment according to the principles of the present disclosure;

FIG. 19 schematically illustrates an example of HARQ retransmissions forthe case when rank changes from 1 to 2 at time of retransmissions asanother embodiment according to the principles of the presentdisclosure;

FIG. 20 schematically illustrates an example of HARQ retransmissions forthe case when rank changes between rank-1 and rank-2 as still anotherembodiment according to the principles of the present disclosure; and

FIG. 21 schematically illustrates an example of HARQ retransmissions ondifferent MIMO layers and different OFDM subbands when MIMO rank changesbetween rank-1 and rank-2 as a further embodiment according to theprinciples of the present disclosure.

DETAILED DESCRIPTION

FIG. 1 illustrates an Orthogonal Frequency Division Multiplexing (OFDM)transceiver chain. In a communication system using OFDM technology, attransmitter chain 110, control signals or data 111 is modulated bymodulator 112 and is serial-to-parallel converted by Serial/Parallel(S/P) converter 113. Inverse Fast Fourier Transform (IFFT) unit 114 isused to transfer the signal from frequency domain to time domain. Cyclicprefix (CP) or zero prefix (ZP) is added to each OFDM symbol by CPinsertion unit 116 to avoid or mitigate the impact due to multipathfading. Consequently, the signal is transmitted by transmitter (Tx)front end processing unit 117, such as an antenna (not shown), oralternatively, by fixed wire or cable. At receiver chain 120, assumingperfect time and frequency synchronization are achieved, the signalreceived by receiver (Rx) front end processing unit 121 is processed byCP removal unit 122. Fast Fourier Transform (FFT) unit 124 transfers thereceived signal from time domain to frequency domain for furtherprocessing.

The total bandwidth in an OFDM system is divided into narrowbandfrequency units called subcarriers. The number of subcarriers is equalto the FFT/IFFT size N used in the system. In general, the number ofsubcarriers used for data is less than N because some subcarriers at theedge of the frequency spectrum are reserved as guard subcarriers. Ingeneral, no information is transmitted on guard subcarriers.

Hybrid Automatic Repeat request (ARQ) is a retransmission scheme wherebya transmitter sends redundant coded information (i.e., subpackets) insmall increments. As shown in FIG. 5, in transmitter 130, an informationpacket P is first input into channel coder 131 to perform channelcoding. The resulted coded bit stream is input into subpacket generator132 to break into smaller units, i.e., subpackets SP1, SP2, SP3 and SP4.The hybrid ARQ retransmissions can either contain redundant symbols orcoded bits which are different than the previous transmission(s) orcopies of the same symbols or coded bits. The scheme which retransmitscopies of the same information is referred to as chase combining. Incase of Chase combining, the subpackets SP1, SP2, SP3 and SP4 as shownin FIG. 4 are all identical. The scheme where retransmitted symbols orcoded bits are different than the previous transmission is generallyreferred to as an incremental redundancy scheme.

An example of Hybrid ARQ protocol is shown in FIG. 3. After receivingthe first subpacket SP1 from transmitter 130, receiver 140 tries todecode the received information packet. In case of unsuccessfuldecoding, receiver 140 stores SP1 and sends a Negative Acknowledgement(NACK) signal to transmitter 130. After receiving the NACK signal,transmitter 130 transmits the second subpacket SP2. After receiving thesecond subpacket SP2, receiver 140 combines SP2 with the previouslyreceived subpacket SP1, and tries to jointly decode the combinedinformation packet. At any point, if the information packet issuccessfully decoded by indication of a successful Cyclic RedundancyCheck (CRC) check, for example, receiver 140 sends an ACK signal totransmitter 130. In the example of FIG. 3, the information packet issuccessfully decoded after receiving and combining three subpackets,SP1, SP2 and SP3. The ARQ protocol shown in FIG. 3 is generally referredto as stop-and-wait protocol because the transmitter waits for theACK/NACK signal before sending the next subpacket. After receiving theACK signal, the transmitter can move on to transmit a new informationpacket to the same or a different user.

An example of N-channel stop-and-wait (SAW) synchronous Hybrid ARQ(HARQ) protocol is shown in FIG. 4. In the example of FIG. 4, N isassumed to equal to 4. In case of a synchronous HARQ protocol, theretransmissions happen at fixed time intervals. With N=4, if the firstsubpacket is transmitted in time slot 1, the retransmissions of thefirst subpacket can only happen in slots 5, 9 and 13. The number ofprocesses is determined by the time required for ACK/NACK feedback. Whenthe transmitter is waiting for feedback on one HARQ process, thetransmitter can transmit another data packet, such as a secondsubpacket. In case of N-channel stop-and-wait (SAW), N parallelinformation packets can be transmitted via N SAW channels, with each ofthe N SAW channels carrying one packet. One of the benefits of thesynchronous HARQ protocol is that the control information only needs tobe transmitted along with the first subpacket transmission because thetiming of the retransmissions is predetermined. The drawback ofsynchronous HARQ, however, is that the retransmission subpackets cannotbe scheduled at preferable channel conditions because the timing of theretransmission is predetermined. Also, the modulation, coding andresource format cannot be adapted at the time of retransmissionaccording to the prevailing channel conditions at the time ofretransmission.

An example of N-channel stop-and-wait (SAW) asynchronous Hybrid ARQ(HARQ) protocol is shown in FIG. 5. In case of asynchronous HARQ, theretransmission timing, modulation, coding and resource format can beadapted according to the prevailing channel and resource conditions atthe time of retransmission. The control information, however, needs tobe sent along with all the subpackets as shown in FIG. 5. The controlinformation transmission along with each subpacket allows adjusting thetransmission timing, modulation, coding and resources allocated.

Multiple Input Multiple Output (MIMO) schemes use multiple transmissionantennas and multiple receive antennas to improve the capacity andreliability of a wireless communication channel. A MIMO system promiseslinear increase in capacity with K where K is the minimum of number oftransmit (M) and receive antennas (N), i.e. K=min(M,N). A simplifiedexample of a 4×4 MIMO system is shown in FIG. 6. In this example, fourdifferent data streams are transmitted separately from four transmissionantennas. The transmitted signals are received at four receive antennas.Some form of spatial signal processing is performed on ii the receivedsignals in order to recover the four data streams. An example of spatialsignal processing is vertical Bell Laboratories Layered Space-Time(V-BLAST) which uses the successive interference cancellation principleto recover the transmitted data streams. Other variants of MIMO schemesinclude schemes that perform some kind of space-time coding across thetransmission antennas (e.g., diagonal Bell Laboratories LayeredSpace-Time (D-BLAST)) and also beamforming schemes such as SpatialDivision multiple Access (SDMA).

An example of single-code word MIMO scheme is given in FIG. 3. In caseof single-code word MIMO transmission, a cyclic redundancy check (CRC)is added to a single information block and then coding, for example,using turbo codes and low-density parity check (LDPC) code, andmodulation, for example, by quadrature phase-shift keying (QPSK)modulation scheme, are performed. The coded and modulated symbols arethen demultiplexed for transmission over multiple antennas.

In case of multiple codeword MIMO transmission, shown in FIG. 4, theinformation block is de-multiplexed into smaller information blocks.Individual CRCs are attached to these smaller information blocks andthen separate coding and modulation is performed on these smallerblocks. After modulation, these smaller blocks are respectivelydemultiplexed into even smaller blocks and then transmitted throughcorresponding antennas. It should be noted that in case of multi-codeword MIMO transmissions, different modulation and coding can be used oneach of the individual streams, and thus resulting in a so-called PerAntenna Rate Control (PARC) scheme. Also, multi-code word transmissionallows for more efficient post-decoding interference cancellationbecause a CRC check can be performed on each of the code words beforethe code word is cancelled from the overall signal. In this way, onlycorrectly received code words are cancelled, and thus avoiding anyinterference propagation in the cancellation process.

In the 3rd Generation Partnership Project (3GPP) Long Term Evolution(LTE) systems, a maximum of two codewords are used for transmission oftwo, three or four MIMO layers. As shown in FIG. 9, for rank-2 or twolayers transmission, codeword-1 (CW1) is transmitted from Layer-0 whileCW2 is transmitted from Layer-1. For rank-3 or three layers transmissionas shown in FIG. 10, codeword-1 (CW1) is transmitted from Layer-0 whileCW2 is transmitted from Layer-1 and Layer-2. For rank-4 or four layerstransmission as shown in FIG. 11, codeword-1 (CW1) is transmitted fromLayer-0 and Layer-1 while CW2 is transmitted from Layer-2 and Layer-3.

In the 3GPP LTE system, a 3-bit HARQ process identity (ID) is used. Theprocess ID refers to the ID of the channel in the N-channelstop-and-wait HARQ. The 3-bit process ID allows simultaneous operationon eight SAW channels. In the example of FIG. 12, the initial subpacketSP1 is transmitted in subframe#0 on process with process ID 0 (PID=0).The retransmissions SP2 and SP3 are performed in subframe#7 andsubnframe#15. With 8 HARQ processes, the minimum time betweenretransmissions is 8 subframes.

An example of subpackets from two codewords is shown in FIG. 13. Weassume that each codeword consists of four subpackets. The subpacketsare referred as redundancy versions (RV) in the context of circularbuffer rate matching used in the 3GPP LTE system. The subpackets or RVsare transmitted in response to ACK/NACK feedback from the receiver.

An example of HARQ retransmission of the two codewords shown in FIG. 13when rank changes from 2 to 1 at time of retransmission is shown in FIG.14. We assume that the transmission of the subpackets from bothcodewords fails on first attempt. As the rank changes to 1 at time ofsubpacket retransmission, only a single codeword can be retransmitted inrank-1. This is because both subpackets used the same process numberthat is process ID 0 (PID=0) and a single subpacket under a single PIDcan be retransmitted in rank-1. The second codeword transmission has tostart from the beginning by transmission of subpacket SP21 at a latertime. This results in loss of the previously transmitted subpacket SP21in rank-2.

The possible Hybrid ARQ feedback message formats are listed in Table 1.

TABLE 1 Hybrid ARQ ACK/NACK feedback HARQ Feedback CW1 CW2 ACK(0)Negatively Acknowledged NA ACK(1) Positively Acknowledged NA ACK(0, 0)Negatively Acknowledged Negatively Acknowledged ACK(0, 1) NegativelyAcknowledged Positively Acknowledged ACK(1, 0) Positively AcknowledgedNegatively Acknowledged ACK(1, 1) Positively Acknowledged PositivelyAcknowledged

An example of HARQ retransmission when rank changes from 1 to 2 at timeof retransmissions is shown in FIG. 15. Two codewords are transmitted inrank-1 in two subframes because a single codeword can be transmitted inrank-1 in a given subframe. We assume that both the codewords requiresretransmission. We further assume that the MIMO rank changes to a rankgreater than 1 enabling transmission of two codewords. We note that theretransmissions for the two codewords cannot be performed in rank-2because the two codewords neet to be transmitted on different hybrid ARQprocesses.

In the current disclosure, we describe a scheme that allows schedulingretransmissions when rank changes at the time of retransmissions.

In a first embodiment according to the principles of the currentdisclosure, in a rank-2 transmission, the process ID of the second CW islinked to the process ID of the first codeword. This requires indicationof only CW1 PID in the control message during the rank-2 transmissionwhile PID for CW2 is derived from CW1 as shown in Table 2. This schemeallows for HARQ retransmissions when the MIMO rank changes from 2 to 1as shown in FIG. 16. As shown in FIG. 16, the first transmission is arank-2 transmission. In the first transmission, the PID1 for CW1 isexplicitly transmitted, while the PID2 for CW2 is derived from the PID1based on Table 2. In the second transmission (retransmission), the rankchanged from rank-2 to rank-1. In this rank-1 transmission, there is nolink between PID1 for CW1 and PID2 for CW2, and therefore both of thePID1 and PID2 are explicitly transmitted in rank-1. Table 2 is only forrank-2 transmission, instead of rank-1 transmission. Note the number ofavailable process indications in rank-1 is sixteen (16), while thenumber of available process indications in rank-2 is eight (8). ThisHARQ retransmission requires that, however, the PID field in rank-1 is1-bit longer than the PID field in-rank-2. For example, if 3-bits PIDrepresenting CW1 PIDs from 0 to 7 (with CW2 PIDs 8-15 implicitlyderived) is used in rank-2, then a 4-bit PID representing PIDs from 0-15is required in rank1.

In the example of FIG. 16, subpackets from four codewords aretransmitted in two subframes with rank-2 (allows two simultaneouscodeword transmission). Note that in FIG. 16, the linking scheme betweenPID3 for CW3 and PID4 for CW4 is the same as the linking scheme betweenPID1 and PID2. We assume that all the four codewords are negativelyacknowledged and requires HARQ retransmissions. Meanwhile, the rankchanges to 1 and therefore the subsequent subpackets from the fourcodewords are transmitted in four subframes with one subpackettransmitted in each subframe. The subpackets can be retransmitted inrank-1 with one subpacket per subframe because the number of hybrid ARQprocess IDs (PIDs) is 2 times more in rank-1 than in rank-2 (16 PIDs inrank-1 versus 8 PIDs in rank-2). The principles of the currentdisclosure can be extended to the case when more than two codewords aretransmitted simultaneously using multi-codeword MIMO. For example, whenthe number of MIMO codewords is four, a 3-bit process ID can be used forthe four codewords transmission in rank-4, and the subpackets for these4 codewords can be transmitted in rank-1 by providing 4 times more PIDs(5-bits PIDS in rank-1), because there are totally thirty-two (32)channels in the HARQ scheme for the four codewords. Similarly, when 4codewords are transmitted in rank-2 with 2 codewords simultaneouslytransmitted, the PID size in rank-2 can be 4-bits. In the case of rank-2transmission, there are two linking schemes for two pairs of codewords.For example, there is a first linking scheme between PID1 and PID2, andthere is a second linking scheme between PID3 and PID4.

TABLE 2 A scheme linking CW2 PID with CW1 PID PID field CW1 process IDCW2 process ID 000 0 8 001 1 9 010 2 10 011 3 11 100 4 12 101 5 13 110 614 111 7 15

In Table 2, the process ID for CW2 (PID2) is linked to the process IDfor CW1 (PID1) as below:PID2=PID1+8  (1)

Other functions for Hybrid ARQ PID linking between CW1 and CW2 can alsobe used. Another example is shown in Table 3 below where CW2 process ID(PID2) is linked to PID1 as below:PID2=16−PID1  (2)

TABLE 3 A scheme linking CW2 PID with CW1 PID PID field CW1 process IDCW2 process ID 000 0 15 001 1 14 010 2 13 011 3 12 100 4 11 101 5 10 1106 9 111 7 8

In a second embodiment according to the principles of the presentdisclosure, an example of HARQ retransmissions according to theprinciples of the current disclosure for the case when rank changes from1 to 2 at time of retransmissions is shown in FIG. 17. The process IDsuse is assumed to be according to Table 3. We assume four subpacketsfrom four different transport blocks (codewords) are transmitted inrank-1. At the time of retransmission when rank changes to 2, thesubpackets transmitted on PID#7 and PID#8 can be scheduled together inrank-2 as allowed by the mapping in Table 3. The subpackets originallytransmitted on PID#5 and PID#6 cannot, however, be scheduled togetherbecause this combination is not allowed by mapping in Table 3. Note thattwo process indications in the same row in Table 3 is an allowedcombination.

In a third embodiment according to the principles of the presentdisclosure, the process IDs for CW1 and CW2 are derived from a single3-bit field as in Table 4. CW1 uses odd numbers PIDs while CW2 uses evennumbered PIDs. This scheme allows simultaneous scheduling of twosubpackets retransmissions from rank-1 to rank-2, when the PIDs of thetwo subpackets are in the same row in Table 4. As shown in FIG. 18, PID1for CWI (SP11) is 4, and PID2 for CW2 (SP21) is 5. PID#4 and PID#5 arein the same-row in Table 4, and hence the retransmissions of therespective corresponding codewords, CW1 and CW2, can be scheduledtogether when the rank changes from 1 to 2. This scheme does not allow,however, retransmission of subpackets on process IDs when the processIDs are not in the same row. For example, PID#4 and PID#5 are not in thesame row in Table 3, and hence subpackets on PID#4 and PID#5 cannot beretransmitted together in rank-2.

TABLE 4 CW1 and CW2 PIDs with a single 3-bit PID field PID field CW1process ID CW2 process ID 000 0 1 001 2 3 010 4 5 011 6 7 100 8 9 101 1011 110 12 13 111 14 15

In a fourth embodiment according to the principles of the presentdisclosure, a full PID field and a differential process ID (DPID) fieldis used for two codewords transmission. An example with a 1-bit DPIDfield linking CW2 PID with CW1 PID is shown in Table 5. When the DPIDfield is set to ‘0’, CW1 PIDs are even numbered while CW2 PIDs are oddnumbered, as given by the following relationship:PID2=(PID1+1)mod 16, when DPID=‘0’  (3)

When DPID field is set to ‘1’, both CW1 and CW2 PIDs are even numbered.However, PIDs for CW2 are shifted by 2, as given by the followingrelationship:PID2=(PID1+2)mod 16, when DPID=‘1’  (4)

This principle can be further extended by using more than 1-bit for theDPID filed. For example, with 2-bit DPID field, the CW1 and CW2 PIDs canbe linked as belowPID2=(PID1+1)mod 16, when DPID=‘00’  (5)PID2=(PID1+5)mod 16, when DPID=‘01’  (6)PID2=(PID1+9)mod 16, when DPID=‘10’  (7)PID2=(PID1+13)mod 16, when DPID=‘11’  (8)

The larger the DPID field, more flexibility is allowed in hybrid ARQretransmissions when the MIMO rank changes between original transmissionand retransmissions.

TABLE 5 CW1 and CW2 PIDs linking by using a single-bit DPID CW2 processID CW2 process ID PID field CW1 process ID (DPIP = ‘0’) (DPIP = ‘1’) 0000 1 2 001 2 3 4 010 4 5 6 011 6 7 8 100 8 9 10 101 10 11 12 110 12 13 14111 14 15 0

In a fifth embodiment according to the principles of the presentdisclosure, a 3-bit process ID is used for two codewords transmissions(rank-2 or greater) and a 4-bit process ID is used for one codewordtransmission (rank-1). An extra codeword to layer mapping (CLM) bit,however, is used for two codewords transmission. When this bit is set,the bit flips the mapping of codewords to layers as shown in FIG. 19.When CLM bit is set to ‘0’, PID#6 and PID#7, for example, goes onLayer-1 (CW1) and Layer-2 (CW2) respectively according to Table 4. Onthe other hand when the CLM bit is set to ‘1’, PID#6 and -PID#7 goes onLayer-2 (CW2) and Layer-1 (CW1) respectively as shown in FIG. 19. Inthis way, the total number of bits is the same between single codewordand two codewords transmission, that is 4-bits process ID for rank-1 and3-bits process ID+1-bit CLM indication for rank-2 and greater.

In a sixth embodiment according to the principles of the presentdisclosure, two 4-bits process IDs (total of 8-bits) are used for twocodewords transmission while a single 4-bit process ID is used for asingle codeword transmission as given in Table 6. This scheme allows forfull flexibility in scheduling and pairing subpackets at retransmissionswhen rank changes such that at sometimes only a single codeword istransmitted while at other times two codewords can be transmitted. Weillustrate this flexibility by considering the example shown in FIG. 20.We assume that the four subpackets transmitted in rank-2 needretransmissions in rank-1. Since a 4-bit PID is available in rank-1, thefour codewords can be transmitted in four subframes with each codewordsneeds its now PID. The subpacket on processes with IDs 0 and 3 failagain and are retransmitted in rank-2 again. Since each codeword has itsown 4-bit process ID, the subpackets from these two codewords can bescheduled together in rank-2.

TABLE 6 CW1 and CW2 PIDs with 4-bits PID fields Rank-1 Rank-2 Total bits= 4 Total bits = 8 CW1 process IDs CW1 process IDs CW2 process IDs4-bits indicate PIDs 4-bits indicate PIDs 4-bits indicate PIDs from 0-15from 0-15 from 0-15

In a seventh embodiment according to the principles of the presentdisclosure shown in FIG. 21, the retransmissions form the two codewordstransmitted on different layers can be scheduled on different frequencysubbands in OFDM. Initially four subpackets are transmitted on twolayers and two subframes in rank-2. The two of the four subpackets failand are retransmitted on a single layer in rank-1 on two OFDM subbands.A subband consists of multiple OFDM subcarriers in a single subframe.Two new subpackets, SP51 and SP61 are scheduled on two subbands on PIDs0 and 9 respectively. The retransmission for SP32 and SP61 on PIDs 3 and9 respectively are then retransmitted on two layers in rank-2 in asingle subframe. By allowing retransmissions in frequency-domain, twosubpackets can be scheduled simultaneously in a single subframe, thusspeeding up the retransmissions and lowering the packet transmissionsdelays. It is also possible to initiate transmission of two subpacketson different subbands as is the case for subpackets SP51 and SP61scheduled on PIDs 0 and 9 respectively. The same ACK/NACK feedbackstructure as for 2 codeword rank-2 transmission given in Table 1 can beused when two subpackets are scheduled on different subbands. In bothcases, a 2-bit ACK/NACK for the two codewords is needed.

The above embodiments of the principles of the present disclosure, i.e.,the methods of transmitting process indications are only application toasynchronous HARQ transmissions when rank changes between originaltransmission and retransmissions.

While the disclosure has been shown and described in connection with thepreferred embodiments, it will be apparent to those skilled in the artthat modifications and variations can be made without departing from thespirit and scope of the disclosure as defined by the appended claims.

What is claimed is:
 1. An apparatus, comprising: a precoder configuredto map modulated symbols into corresponding layers based on codeword tolayer mapping information and a swap flag; and a processing unitconfigured to transmit the modulated symbols over the mapped layers andto transmit control information, the control information comprisingmodulation information, resources allocation information, precodinginformation, and the swap flag.
 2. The apparatus of claim 1, wherein anumber of the layers allows a determination, from the codeword to layermapping information, of a mapping relationship between codewords andlayers.
 3. The apparatus of claim 2, wherein the swap flag indicatesthat the mapping relationship between transport blocks and codewordsshould be swapped.
 4. The apparatus of claim 2, wherein the controlinformation further comprises a hybrid automatic repeat request (HARQ)process identity.
 5. The apparatus of claim 1, wherein the controlinformation further comprises information indicating a change of anumber of layers during retransmission.
 6. A method, comprising:mapping, using a precoder, modulated symbols into corresponding layersbased on codeword to layer mapping information and a swap flag; andtransmitting, from a processing unit, the modulated symbols over themapped layers and control information, the control informationcomprising modulation information, resources allocation information,precoding information, and the swap flag.
 7. The method of claim 6,wherein a number of the layers allows a determination, from the codewordto layer mapping information, of a mapping relationship betweencodewords and layers.
 8. The method of claim 7, wherein the swap flagindicates that the mapping relationship between transport blocks andcodewords should be swapped.
 9. The method of claim 7, wherein thecontrol information further comprises a hybrid automatic repeat request(HARQ) process identity.
 10. The method of claim 6, wherein the controlinformation further comprises information indicating a change of anumber of layers during retransmission.
 11. An apparatus, comprising: areceiver configured to receive modulated symbols over mapped layers andto receive control information, the control information comprisingmodulation information, resources allocation information, precodinginformation, and a swap flag; and a processor configured to determinethe modulated symbols from mapping of the modulated symbols tocorresponding layers based on codeword to layer mapping information andthe swap flag.
 12. The apparatus of claim 11, wherein a number of thelayers allows a determination, from the codeword to layer mappinginformation, of a mapping relationship between codewords and layers. 13.The apparatus of claim 12, wherein the swap flag indicates that themapping relationship between transport blocks and codewords should beswapped.
 14. The apparatus of claim 12, wherein the control informationfurther comprises a hybrid automatic repeat request (HARQ) processidentity.
 15. The apparatus of claim 11, wherein the control informationfurther comprises information indicating a change of a number of layersduring retransmission.
 16. A method, comprising: receiving, a receiver,modulated symbols over mapped layers and control information, thecontrol information comprising modulation information, resourcesallocation information, precoding information, and a swap flag; anddetermining, using a processor, the modulated symbols from mapping ofthe modulated symbols to corresponding layers based on codeword to layermapping information and the swap flag.
 17. The method of claim 16,wherein a number of the layers allows a determination, from the codewordto layer mapping information, of a mapping relationship betweencodewords and layers.
 18. The method of claim 17, wherein the swap flagindicates that the mapping relationship between transport blocks andcodewords should be swapped.
 19. The method of claim 17, wherein thecontrol information further comprises a hybrid automatic repeat request(HARQ) process identity.
 20. The method of claim 16, wherein the controlinformation further comprises information indicating a change of anumber of layers during retransmission.